Spherical double reflector antenna

ABSTRACT

There is described a double reflector antenna arrangement having a fixed spherical-segment main reflector and at least one spherical subreflector surface concentrically arranged relative to the main reflector, which spherical/spherical double reflector antenna is particularly useful as a non-moving communication antenna capable of operating with one or more synchronous satellites. Multiple beam operation is effected by providing a feed with each spherical subreflector, wherein the position of the feeds relative to the main and subreflecting surfaces is optimized as a function of the antenna F/D ratio and especially the associated subreflector radius as defined by the spherical center point of the main reflector.

Unite States Patent [191 Bresler et al.

[ Dec. 3, 1974 SPHERICAL DOUBLE REFLECTOR ANTENNA Primary ExaminerPaul L. Gensler [75] Inventors: Aaron D. Bresler, Merrick, N.Y.; 1x522 21 2; f fl O Hallow;

Emanuel Stein, Fair Lawn; M. Otto E d D r mann Denvrlle both of N J ABSTRACT 73 Asslgnee: Intematmnal Telephone and There is described a double reflector antenna arrange- Telegraph Corporaflon, Nutley* ment having a fixed spherical-segment main reflector [22] Filed: Dec. 19, 1972 and at least one spherical subreflector surface concentrically arranged relative to the main reflector, which [21] Appl' 316617 spherical/spherical double reflector antenna is particularly useful as a non-moving communication antenna 52] us. Cl 343/781, 343/837, 343/912 capable of Operating with one or more Synchronous 51 Int. Cl. H01q 19/14 Satellites. Multiple bfiam Operation is effected y p [53 Fi l f Search 343/ 1 2 775 779 731 viding a feed with each spherical subreflector, wherein 343 731 A, 7 C, 3 337 40 the position of'the feeds relative to the main and subreflecting surfaces is optimized as a function of the an- 5 References Cited tenna F/D ratio and especially the associated subre- UNITED STATES PATENTS flector radius as defined by the spherical center point of the main reflector. 3,406,40l 10/1968 Tillotson 343/779 X 3,423,756 1/1969 Foldes 343/775 18 Claims, 12 Drawing Figures l i i I {x :COS 6 SIN e #4 s A 2e(&i )1

R I i /v APERI'URE ,SPHER/CAL I, I PLANE 5 REFLECTOR I sufg iiiiofi/ I z 4 a P I E l Hfi 1 9 6A 51/ v w! 9 F v T I x =O W X=P" 51/ =l PATENTEL E sum 1 UF 8 Qokuw vwww PATENTEL W4 SHEET 2 BF 8 :pzso APERTURE r0 suansnscron VRTEX SPAC/Nq PATENIELB'EB 31914 3.852.765

SHEET 3 OF 8 gfiigag OPT/MUM FEED-HORN APERTURE LOCATIONS 0 ol? ZR0 PA TH uwar/x ERROR Ar APERTURE EOGE F/D RAT/O PATENIEL 31974 3,852,765 SHEET 50$ 8 age .5

S O R AA m m MM RWK o A A M 3 m: s R 5 A C II I l I l 1 l II III F A Z S R s .w w m U C M R m Wm... U o $0..-IAI. a" M H E OE C R E Ul- R s E E R W R NR 0 A a a r M a m m S S 0 AL a is an A 5 MR 9 w I $15.... .51 T P O 5 O 5 O 3 a 2 4 1 m M to d SPHERICAL DOUBLE REFLECTOR ANTENNA BACKGROUND OF THE INVENTION This invention relates to double reflector type antenna arrangements with spherical main reflectors, and more particularly to double-reflector spherical antenna arrangements in which the subreflectors are spherical. The invention embraces all applications involving antennas for electromagnetic waves. However, particular reference is made in this disclosure to satellite communications, with major emphasis in the given example embodiments of this invention being described in reference thereto.

Domestic type communication satellite systems are currently under consideration in the United States and many other countries for the distribution of television, voice and data over widely dispersed locations within a geographical entity. Typical frequency ranges of operation involved in commercial satellite communications are the 500 MHz receive band running from 3.7 to 4.2 GHz and the 500 MHz transmit band running from 5.925 to 6.425 GHz. There is no limitation to use in other frequency bands, such as those currently allocated for satellite communications in the SHF band. For this application, there is need for low-cost limitedmotion antennas to operate with unattended or semiattended terminals and be capable of simultaneous operation or quick switchover to a backup satellite (or satellites), with minimal or preferably no transmission outage and interruption of traffic.

Currently proposed domestic systems require switchover between satellites (within the synchronous equatorial plane), spaced anywhere from 4 for sun outage control (i.e., satellites within small angular relationship to the sun experience greatly increased sun noise, making communications reliability suffer considerably) through 50 for east-west traffic switchover. Satellite selection may be implemented through switchover between prepositioned separate antennas, switchover between prepositioned multiple feeds within a common reflecting geometry, or preset mechanical positioning of a single antenna either manually or by motor drive. The first and third methods mentioned have several disadvantages including the limitations of high cost or the possibility of significant traffic interruption.

In addition, a number of proposed domestic satellite systems include simultaneous operation of an earth station with more than one satellite.

The relatively large number of antennas required for future domestic systems around the world dictates the need for economically producible and easily erectable designs for US. and foreign applications.

There are a number of known double-reflector systerns which can be used to achieve multiple-beam operation. The known systems include some which employ a spherical main reflector. Thus the use of steerable beam, fixed spherical reflector antennas is not new. Spherical reflector movable-feed antennas, 12 or more feet in diameter, have been used in some rapid-scan radar and radiometer applications. A 1,000 foot spherical reflector having a movable feed that can steer the beam plus or minus in any direction has been operated successfully for several years in Puerto Rico for radio-astronomy applications.

Parabolic-Cassegrain configurations for small angle separation situations have been suggested, designed for off-axis operation of dual beams with separation angles up to a maximum of 8. This provides an economical approach to limited separation, dual beam operation using single main reflector/subreflector geometry with two feeds. This economy, however, is achieved at the expense of progressive degradation in gain and side lobe performance as the offset angle is increased. Moreover, the limited separation capability would make such configurations inapplicable in wider separations contemplated in satellite communications.

For the satellite operations contemplated, there is desired a variable beam separation capability encompassing beam separations of 2 to greater than 40. The most probable look-angle (elevation) for most satellite applications is between 20 through however, it is desired that an antenna be capable of being set up for operation anywhere in the range of 5 to The satellite antenna arrangement must be capable of operation under environmental conditions anywhere on earth, primarily at the satellite frequencies given above, but having the adaptability for operation at fre quencies up to 31 GHz using beamwidths typically from 05 to 0.2, and not pose any fundamental limitations excepting tighter tolerances of surface and positioning accuracy.

Economic considerations are of major importance in determining the optimum antenna configurations for the above. As a target, the arrangement of both feed and reflectors must be capable of being implemented through relatively simple manufacturing techniques and use of available materials, require non-critical positioning tolerances/surface accuracies, and be easy to assemble in the field.

In considering spherical antenna arrangements to meet this challenge, it is noted that a particular trouble with spherical reflectors is that they really do not have a clearly defined focal point. The paraxial focus is merely the vertex of the caustic surface and therefore represents the point of maximum energy density. Thus, the excitation of a spherical reflector by a single point source feed always results in a certain amount of square-law phase error.

Devices designed to correct the square-law phase error include the use of phased line sources and correcting reflectors. However, these tend to be very long narrow-band devices not suitable for present applications which require wide band operation particularly at 4 and 6 GHz.

When a spherical reflector is used with a single point source feed the usable aperture diameter is limited by the allowable phase error.

There have been achieved in the art quite narrow beams (satisfactory narrow beamwidth performance) with short focal length spherical reflectors by using a subreflector which completely corrects the aperture phase errors. The required subreflector shape is defined by a trigonometric equation which provides an exact correction for the path lengths for all rays contained within a specified cylindrical diameter. The required subreflectors are, however, not spherical. Once the diameter of the aperture and the nominal focal length (i.e., spherical systems raduis) have been specitied, the required subreflector diameter has a minimum size which, for good designs, limits the minimum beam separation to no less than perhaps 8. Since the prior art subreflectors are not spherical, they cannot be .blended together to accommodate closely spaced beams.

SUMMARY OF THE INVENTION A principal object of this invention is therefore to provide a'spherical/spherical double-reflector antenna arrangement and method for implementation.

It is also an object of this invention to provide an op timum configuration for a non-moving reflector antenna capable of operating with one or more synchronous satellites by means of multiple feeds.

Another object of this invention is to provide a spherical double-reflector antenna design intended to apply the advantages of spherical reflector antennas to communication satellite systems. This objective is obtained through the addition of a concentric spherical subreflector, which makes the spherical antenna geometry physically compact and more adaptable to small earth stations, reduces aperture phase error, minimizes system noise contribution due to spillover and permits the use of small aperture, readily available feeds.

It is a further object of this invention to provide an optimum configuration for a non-moving reflector communications antenna capable of operating with one or more synchronous satellites either simultaneously or by means of switching multiple feeds, wherein the main reflector, which determines the antenna aperture, remains fixed.

1 It is yet another object of this invention to provide an antenna arrangement fully compatible with wide angle (beam separation up to 50 or more) and limited angle (beam separation from 28), multiple beam satellite system requirements, and using materials and fabrication techniques suitable for low cost manufacturing/as sembly.

The prior art arrangements require separate antennas for each beam or require the movement of a single beam from one satellite to pick up the other satellite. which necessitates interruption of continuous operation and traffic flow. It is therefore yet a further object of this invention'to provide a highly reliable and stable multiple beam antenna using a minimum of moving parts having a capability for implementation of either instantaneous satellite switchover (in the event of sun outage or satellite failure) while assuring continuous operation, or simultaneous two-satellite operation (dual beam) at minimum cost.

lt is another object to provide a multiple-beam domestic earth station antenna arrangement particularly for satellite communication according to the specifications given hereinbefore, while avoiding or eliminating the prior art drawbacks above-mentioned.

A further object is to provide an antenna arrangement with the above objects in mind which is capable of being implemented through relatively simple manufactoring techniques and use of available materials, and which requires non-critical positioning tolerances and which will be easy to assemble in the field.

It is another object to provide an antenna arrangement for satellite operations which is capable of implementing such operation by the unique tilting of the sub reflecting surfaces only rather than the main reflector, or by the movement only of the feeds wherein the main reflector and subreflectors are held fixed.

According to the broader aspects of this invention there is provided an antenna arrangement comprising a spherical main reflector surface, a spherical subreflector surface having a radius of curvature origin coincidint with that for the main reflector, and at least one feed predeterminably arranged relative to said main and subreflector surfaces.

The antenna system consists ofa concentric spherical segment main reflector and one or more spherical subreflectors, with a number of feeds equal to the required number of beams. One configuration proposed is basically a double reflector system, including a spherical subreflector, concentric with the main spherical reflector, in a Cassegrain type configuration.

It is not immediately evident, even to one skilled in the art, that a spherical/spherical double-reflector antenna system is a feasible solution to the multiple-beam antenna problem, that there exists an optimum feed horn location for any given subreflector radius, and that with this optimum subreflector and feed horn location, the phase error efficiency of the double-reflector system exceeds that of a single reflector system employing the same spherical main reflector.

A spherical reflector has no preferred axis; therefore, it can remain fixed and simultaneously provide multiple beams by the addition of feeds suitably positioned. In other words, a single reflector area is capable of being shared by multiple beams without significant degradation, when properly designed for low blockage and scattering effects.

Individual beam steering or tracking may be easily implemented by positioning the'corresponding feed relative to the main spherical center in any direction. A spherical design eliminates the need for high precision positioning oflarge mass structures, and substitutes positioning of relatively small, low-mass feed assemblies.

Large spherical surfaces can be physically subdivided into a grindwork of similar panels, each having identical surface curvature, but slightly tapered sides (in longitude direction). This lends itselfto the application of inexpensive fabrication techniques (using common tooling) to quantity produce large numbers of similar panels of a relatively high surface accuracy.

Therefore, in further accordance with the broader aspects of this invention there is provided an antenna assembly comprising a fixed spherical main reflector of predetermined physical dimensions composed of a multiplicity of reflecting panels having substantially identical spherical reflecting surface curvature arranged in mosaic form to effect a substantially continuous spherical main reflecting surface, at least one spherical subreflector mounted to be concentrically arranged relative to said main reflector at a predetermined separation therefrom, and a separate feed for each subreflector predeterminably mounted relative thereto to form a feed/subreflector assembly arrange ment requiring only feed positioning to implement beam steering and tracking.

Moreover, the invention provides fora method of optimizing a spherical/spherical double-reflector rediating arrangement, comprising the steps of determining,

- in relation to a spherical main reflector, the position of a concentrically arranged spherical subreflector in terms ofthe spherical center point defined by the main reflector and placing the subreflector thercat; and determining the feed location relative to the vertex of the spherical subretlector for a given F/D ratio so as to provide substantially zero path-length error at the aperture edge of the spherical main reflector.

Included also as a part of this invention is a method of constructing a spherical reflector antenna comprising the steps of molding individual reflector panels to have substantially identical spherical reflector surface curvature; adjustably assembling the panels in a matrix arrangement on a fixed support structure to form a main antenna reflector surface; and aligning each panel to achieve a substantially continuous spherical reflector surface.

Some of the advantages and features of this invention are given in the following.

Each reflecting surface segment of the main reflector is a spherical section with a single radius of curvature, i.e., equal radii of curvature for all segments of each reflector surface so that, once a set of design parameters have been selected, the same basic tooling can be used to fabricate reflector segments for any antenna size and shape.

The basic design parameters are independent of the number of beams required and of their angular separation so that, for example, as the required number of beams or the required beam separation increases, the antenna system can be expanded to meet the increasing requirements without changing the basic design or toolmg.

The phase error efficiency using the double reflector system is higher than for single spherical reflector systems of the same aperture and F/D ratio.

Although the present disclosure describes antenna systems designed for satellite communications with the multiple beams disposed in a single plane of scan, the basic design is not limited to wide-angle scanning in one plane (as is the case with for example the parabolic torus). The basic configurations described herein are capable of providing beam steering in any plane without degradation of performance.

In the design criteria discussed hereinafter there is described a method for selecting anoptimum separation between feed horn and subreflector for each combination of subreflector and main reflector radius. With the feed horn at its specified optimum location, the phase-error efficiency of the double-reflector antenna system is greater than that of a single reflector antenna consisting of the same spherical main reflector and an optimally located feed horn.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other objects and features of this invention will become more apparent and the invention itself will be best understood by reference to the following description taken in conjuction with the accompanying drawings, in which:

FIGS. 1A and IB illustrate the geometry and coordinate definitions for analyzing the aperture plane phase errors of a spherical reflector with point source feed;

FIG. 2 illustrates the geometry and coordinate definitions for a spherical/spherical double reflector antenna system;

FIGS. 3 and 4 represent charts of optimum feed horn aperture locations and their associated maximum path length errors;

FIG. 5 illustrates geometry explaining the relationship for the maximum feed horn diameter to achieve equal feed horn and subreflector blocking;

FIG. 6 illustrates variation of reflector system surface areas as a function of beam separation angle for a two beam system;

FIG. 7 illustrates the estimated variation in blocking efficiency for a typical dual beam spherical doublereflector design, as a function of beam separation an- FIGS. 8A and 8B illustrate in cross-sectional views two spnerical/spherical double-reflector antenna configurations for dual beams (one at 10 separation using an elongated spherical subreflector, and the other at 20 separation using separate spherical subreflectors), and the relative positioning of the feed and subreflector with respect to the main spherical reflector; and

FIGS. 9A and 9B illustrate in perspective an example embodiment of a double beam spherical antenna configuration according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In arriving at an optimum configuration for a spherical double reflector antenna capable of providing multiple beams separations over approximately 270, a configuration of particular interest is a spherical torus main reflector with a Cassegrain type spherical torus subreflector whose radius of curvature origin is coincident with that for the main spherical system. This provides a capability of continuous beam scanning in the orbital plane (hour/angle rotation) over the full range indicated merely by positioning one or more feeds along a circular are without necessity of moving either the main reflector or sub-reflector (concentric spherical surfaces).

In theory, it should be possible to replace the combination of point source feed and subreflector by an equivalent image source which, to a first approximation, would be an equivalent point source. It is, therefore, advisable to examine the aperture plane phase errors which result when a spherical reflector is illuminated by a point source feed.

Reference is made to an analysis of the spherical reflector with point source feed given by Mr. T. Li in A Study of Spherical Reflectors as Wide Angle Scanning Antennas, Trans. IRE-AP, 7, number 3, pp. 223-226, July 1959. The geometry and coordinate definitions for this are given in FIGS. 1A and 1B. The approximation v==20 shown therein is inte n ded t o imply thaiwhen calculating the path length, FS SA. the ray SAT is assumed to be parallel to the X axis. Subject to this approximation, the normalized path length error is given y A/R l-l (P8 SA) (FP PQ) The information given in FIGS. 1A and 1B shows that, for a feed horn located at a normalized distance f 0.5 from the reflector vertex, the path length error versus angle 6 first increases slowly to a positive maximum and then decreases rapidly through zero to large negative values. An optimum feed location is one which locates the zero path length error at the aperture edge. Thus the information given in FIGS. 1A and 18 makes evident that, for a given nominal F/D ratio (F/D R/2D O.25/sin0, there is a uniquely defined optimum feed location and a uniquely defined associated maximum path length error. With increasing F/D ratios, the optimum feed location moves closer ,to the paraxial focus and the maximum path length error decreases. An analogous situation exists for the spherical/spherical double reflector antenna.

The geometry and coordinate definitions for this latter antenna are given in FIG. 2. The antenna consists of a spherical main reflector with normalized radius R l and a concentric spherical subreflector with normalized radius p. The subreflector is illuminated by a feed horn located a distance from the subreflector vertex. A typical ray (shown leaving the feed at an angle (11 to the principal axis) is not, in general, parallel to the principal axis after reflection from the main reflector at point S. However, for the range of parameters which are of interest here, this angle is typically less than 1. Therefore, for path length calculations, the ray segments SA is assumed to be parallel to the X axis. Subject to this approximation, the path-length error is given by The complete set of formulas required for the calculation of the path-length error is given below in terms of the parameters p and (r and the angles 6, I and 4).

2 1- 4psin 1/z( A/R (a 0') [/3 (l p)] {cos 6 COSth/Cos [26 (2? (l C080 (ozo')+[,B(l 'O')](l @050) (where20-- 2I' 4)) The complexity of these expressions makes it difficult to state general conclusions other than by resorting to extensive calculation. However, it has indeed been I found that the variation of path-length error as a function of the aperture coordinate 0 is as depicted in FIGS. 1A and 1B. In the case of the double reflector antenna, the optimum separation of the feed horn from the subreflector vertex is a function of both the F/D ratio and the subreflector radius. Selected typical optimum feed locations and the associated maximum path length errors are charted in FIGS. 3 and 4 for the range of parameters of interest here.

Examination of FIG. 3 shows that the optimum feedhorn to subreflector spacing, a, is primarily a function of the subreflector radius and exhibits only a second order dependence on the F/D ratio. As p is decreased, the optimum 0' value decreases and approaches zero as p approaches 0.5.

FIG. 4 shows that the maximum path-length error is primarily a function of the F/D ratio and exhibits only second order dependence on p. As p is decreased toward 0.5, the maximum path-length error approaches that of the spherical reflector with optimum-location point source feed.

From the data given in FIGS. 3 and 4 above, one might conculde that an optimum design would employ as large a value ofp as possible in order to minimize the path-length error and to locate the feed close to the main reflector vertex. However, increasing p also increases the subreflector-to-main reflector diameter ratio and therefore increases the blocking loss. The blockingdiameter ratios seem to be primarily a function of p and to exhibit only a second order dependence on the F/D ratio. Thus, the maximum allowable blocking loss sets an upper limit for the usable p values.

A lower limit to the usable p values is obtained by requiring that the feed-horn blocking shall not exceed that of the subreflector. The geometry associated with the imposition of this requirement is shown in FIG. 5, where 8 represents the maximum allowable feed-horn diameter for equal feed-horn and subreflector blocking. It can be shown that for a fixed F/D ratio, 8 decreases as p decreases, and eventually becomes less than the minimum feed-horn diameter required to provide some specified value of subreflector illumination taper. The value of p should be selected to be somewhat larger than its value at this crossover point to allow room for a horn ring.

The desirable design goals include achievement of the required performance with minimum F/D ratio ad minimum reflector diameter. This will minimize the reflector surface area required for a multiple-beam torus design. The example best-compromise design described herein employs a main reflector having an aperture D 36 feet and R 54 feet (F/D 0.75).

The major performance change which occurs when the single-beam design is developed into a toroidal multiple-beam design is the increase in the aperture blocking losses. There are, of course, other small changes such as a redistribution of the subreflector spillover and diffraction energies, small beamwidth changes, small increases in side lobe levels, etc. These second-order effects are not materially significant and are ignored in the considerations here.

For closely spaced beams, the increased subreflector width will result in the illumination of an aperture area which is larger than that of the basic single-beam antenna. However, the illumination of this additional area will be very inefficient (low phase-error efficiency) and therefore, the possible increase in aperture gain is also ignored in the discussion which follows.

FIG. 6 shows the variation of reflector system surface areas as a function of beam separation angle for a twobeam system. The estimated variation in blocking efficiency for this case is shown in FIG. 7.

The results given in FIG. 7 indicate that, with separate subreflectors for the individual beams, the maximum increase in blocking loss for a two-beam system over a single beam system is about 0.25db. For a threebeam system the maximum increase in blocking loss (with separate subreflectors) is about 0.5 db.

FIG. 8A illustrates in cross-sectional view a multiplebeam double reflector antenna system consisting of a spherical-segment main reflector and a spherical subreflector concentric therewith and a pair of feed horns (one for each beam). As may be seen in FIG. 8A, for small angle beam separations (i.e., small 6) the spherical subreflectors for the adjacent beams overlap. This overlap is readily accommodated since the individual subreflectors are both segments of equal radius spheres. For small angle separations, the preferred configuration, therefore, consists of a spherical torus main reflector and a concentric spherical torus subreflector; a continuous band up to 9.5 spacing, and separate spherical subreflectors for each feed from 9.5 to maximum beam separation (50 in this case).

Design criteria have been given which show how to select the reflector radii to meet specified antenna performance requirements. These design criteria include the definition of an optimum separation between feedhorn and subreflector for each combination of subreflector and main reflector radius. With the feed horn at its specified optimum location, the phase-error efficiency of the double reflector antenna system is geeater than that of a single-reflector antenna consisting of the same spherical main reflector and an optimally-located feed horn.

Some of the advantages of the spherical/spherical double reflector system described are:

Each reflecting surface is a spherical section with a single radius of curvature, i.e., equal radii of curvature for all segments of each reflector surface so that, once a set of design parameters have been selected, the same basic tooling can be used to fabricate reflector segments for any antenna size and shape.

The basic design parameters are independent of the number of beams required and of their angular separation so that, for example as the required number of beams or the required beam separation increases, the antenna system can be expanded to meet the increasing requirements without changing the basic design or tooling. Note in FIG. 8A that for each additional beam, it is intended to add a feed horn, separate subreflector or subreflector extension, and main reflector extension.

Although the present disclosure describes an antenna system designed for satellite communication with the multiple beams disposed in a single plane of scan, the basic design is not limited to wide-angle scanning in one plane (as would be the case with for example the parabolic torus). The basic configurations described herein are capable of providing beam steering in any plane without significant degradation of performance.

FIG. 88 illustrates a specific double reflector configuration of FIG. 8A which includes a spherical subreflector concentric with the main spherical reflector in a Cassegrain type configuration, with, however, a separate subreflector for each feed illustrative of the wider beam separation capabilities, according to the invention. Though specific references to structure herein are limited to a pair of feeds in one-to-one correspondence with a pair of subreflectors, it is to be understood that the scope of this invention includes any suitable number N of corresponding feeds and subreflectors.

A basic structural representation is given in FIGS. 9A and 9B for a specific dual-beam spherical antenna depicted in FIG. 88 with a satellite separation of roughly 18 in longitude. The configuration in FIGS. 9A and 9B is a highly reliable and stable multiple beam antenna (using minimum moving parts) with an ingerent capability for implementation of either instantaneous satellite switchover (in the event of sun outage or satellite failure) while assuring continuous operation, or simultaneous two-satellite operation (dual beam) at minimum cost. The particular arrangement presented in FIG. 9A assumes for this discussion that the two satellites are positioned typically at and 108 west longitude respectively. The proposed configuration consists of a single fixed 60 X foot (nominal) spherical main reflector I04 composed of approximately one hundred similar panels with a separate feed/subreflector assembly for each satellite (of which only one is shown in FIG. 9A for clarity). Nominal elevation X of the main reflector support is typically 45.Two separate low-blockage support columns 100, 101 are provided to individually locate each feed/subreflector assembly relative to the main reflector 104. The basic configuration for the dual-beam spherical antenna given in FIG. 9A includes the location for the second subreflector/- feed tower 101 to provide a nominal 18 beam separation in longitude. The configuration geometry depicted requires only feed positioning to be implemented for beam steering or automatic tracking. Independent closed loop servo control (using signal strength input data) is applied to a two-axes positioner for each feed 102 and 103 (not shown) to provide automatic tracking for satellite drifts up to i0.25 in both longitude and orbital inclination. In this basic geometry consisting of the spherical main reflector 104 illuminated by a Cassegrain type feed/spherical subreflector assembly (102,107), since both the main reflector 104 and subreflector 107 are concentric spherical surfaces (relative to a common spherical center as shown in FIG. 8B), beam pointing is a function of the position of the feed 102 on a spherical radius relative to the same spherical center (FIG. 88). Due to symmetry, beam pointing is equally effective in any direction, provided that the rays do not fall beyond the physical boundaries of the main and subreflector surfaces.

The implementation of a spherical antenna design presents technical and economical advantages as indicated in the following.

A spherical reflector has no preferred axis; therefore, it can remain fixed and simultaneously provide multiple beams by the addition of feeds suitably positioned (as feeds are added, blockage per beam may increase requiring a corresponding increase in spherical aperture to meet a given antenna gain). A single reflector area is capable of being shared by multiple beams without significant degradation, when properly designed for low blockage and scattering effects.

, Individual beam steering or tracking can be easily implemented by positioning the corresponding feed relative to the main spherical center in any direction. A spherical design eliminates the need for high precision positioning of large mass structures (such as parabolic reflectors), and substitutes positioning of relatively small, low-mass feed assemblies.

Large spherical surfaces can be physically subdivided into a gridwork of similar panels, each having identical surface curvature, but slightly tapered sides (in longitude direction). This lends itself to the application of inexpensive fabrication techniques (using common tooling) to quantity produce large numbers of similar panels of relatively high surface accuracy (same template and-measurement procedures are used for all panels). As a consequence of standardization, the panel size is optimized to permit maximum fabrication flexibility with ease of handling in-plant, during shipment, and on-site during assembly/erection.

The technique of constructing spherical surfaces through the assembly of standard panels (each capable of being adjusted separately) onto a fixed billboard board type support structure, simplifies the on-site assembly and alignment of relatively large reflector surfaces. In addition, this type of construction permits the surface area to be varied to meet a large range of required antenna aperture and beam spacings, through the addition (or removal) of panels onto the support structure.

The dual beam spherical antenna is also advantageous in its foundation requirements. Unlike the movable parabolic reflector, which concentrates the ground reactions to within a relatively small area, the fixed billboard type of construction spreads the earth reactions over a large ground surface, resulting in high resistance to overturning moments, with relatively small unit stress. This permits the application of economical flat-slab type construction where large area soil cover is used to maximum advantage.

As stated, the autotrack system for positioning the feed uses a closed loop servo control wherein each feed is capable of tracking its own satellite over maximum range of 10.25" in orthogonal directions, corresponding to the satellite drifts in longitude and orbital inclination. The small mass of the feed permits the elimination or significant reduction in the "size of the motor control,

unit, lowering the cost and improving reliability.

Cost effectiveness is maximized by providing that the arrangement implement the capability of either instantaneous satellite switchover or simultaneous twosatellite operation.

The proposed double spherical antenna system shown in FIGS. 9A, 9B consists of the following major subassemblies:

mainspherical reflector 104 (consisting of 104 panels) fixed support structure 105, including panel adjustment mechanism foundation 106 spherical subreflectors 107 and 108 (two for dualbeam operation, but only one shown here for clarity) subreflector/feed support towers 100 and 101 including electronic equipment enclosures 110 Note that the subreflector and feed support and associated adjustment mechanisms may also be implemented through the use of conventional quadrapod, tripod, or bi-pod structures attached to the main reflector structure and suitably guyed. The choice of ground towers or reflector supported members is dependent only upon achieving low blockage with high stability in any particular application.

feeds and positioners 102 and 103 (two for dualbeam operation but only one shown) The main reflector assembly 104 is made up of 104 plastic skin-steel reinforced panels, each with five ad-' justment support points through which it can be affixed to the support structure. The panels are arranged in a matrix measuring eight panels high by 13 panels wide. When assembled and aligned, theyform a section of a sphere measuring nominally 60 feet high by 100 feet wide in this example embodiment. Each panel thus measures approximately 7.5 feet in height by 7.7 feet in width. It is estimated that the total weight of the main reflector for this application would be in the vicinity of 16,000 pounds. Each panel is designed to minimize distortion under worst conditions of operating environmental conditions, including wind and thermal gradients, and contains adequate internal stiffening to provide adequate structural stability.

All panels are constructed using a molded fiberglass reinforced laminated ployester skin, rigidized by a steel framework backing. An r.f. conductive coating (e.g.. 0.005 inch thick aluminum) covers the smooth reflec tive side of the panel. Each panel is intended to have a maximum tolerance of 0.02 inch rms relative to thc true spherical surface. When completely assembled, the main reflector is intended to have a static no-load surface accuracy of 0.04 inch rms over any 60 foot diameter aperture. Under normal environmental operating conditions, the reflector is intended to have an operating surfaceaccuracy of 0.06 inch rms maximum.

The fixed steel structure which supports the main reflector assembly consists of a number of main A frames interconnected by steel framework which closely hugs the spherical surface.

The reflector surface 104 is affixed to the support structure 105 by means of mechanical pick-up points (not particularly shown capable of being adjusted to the required degree of surface accuracy, and then locked into position. Each individual panel is rigidly supported to the support structure at five points, located near each of the panel corners and at the center. The panel adjustment mechanism as part of the support structure, allows each panel individually to be adjusted in pitch, yaw and surge (referenced to the focal axis), introducing negligable panel distortion during adjustment. Lock pins are used to lock each panel in place after final alignment. Also, during the initial trial assembly in-plant, it is intended that these lock pins will bedrilled in place, and serve as index references during reassembly on site.

At an earth station site, the fixed support structure must provide correct antenna reflector orientation to operate simultaneously with two'satellites at the specified orbital positions. A further consideration in the positioning of a fixed reflector for multi-beam operation is the degree of reflector scan plane rotation, or tilt, required to efficiently illuminate two or more beams at different relative elevations. Such a condition arises for instance where the dual satellites are near the horizon relative to the antenna assembly. The differential elevation angle for dual, or multi-beam operation at any site can be implemented by either rotating, or tilting the antenna scan plane to the indicated tilt angle (thereby maintaining a minimum reflector surface area as dictated by the performance requirements), or increasing the reflector surface area to accommodate the differential beam offset without mechanically tilting the reflector, or a best combination of both. Analysis indicates that up to 10 of antenna structure tilt relative to grade can be reasonably accommodated without requiring structural modification.

Particular positioning requirements of the feeds and subreflectors for this antenna system are optimally met through the use of a shaped guyed tower (100,101) (slender in the direction of radiation) to support the subreflectors (107,108), the feed and-feed positioners (102 and 103), and the electronic equipment assembly 112 enclosed in a protective housing behind the subreflectors 107 and 108. In providing dual-beam op- From FIG. 83 it may be seen that each subreflector consists of a spherical surface having a diameter Y (nominally 8.4 feet and radius of curvature of 57 feet). lt is intended that the subreflector be fabricated from fiberglass reinforced polyester with a maximum of 0.20 rms surface tolerance including the conductive aluminum surface layer. Means for manual adjustment are provided with up to five degrees of freedom to permit optimum positioning during antenna alignment.

The feed for the spherical double reflector configuration is basically of reduced size as to the horn section (60 primary illumination) to give a smaller and lighter unit (e.g., overall length of 60 inches).

As stated, in the spherical double reflector configuration, only the feed is required to move during beam pointing and tracking. For this design, with a required beam steering within a i0.25 range (in both longitudinail and orbital inclination directions), the calculated phase center of the feed aperture must rotate on about a 64.5 foot radius around the spherical center. This represents an essentially linear motion of approximately 5 inches, at right angles to the spherical center radius. Other applications will require other excursions of travel.

The feed positioning is accomplished through mounting the feed to an X-Y planar linear positioner capable of two-direction motion. The positioner consists of two independently driven linear actuators (not shown) mounted orthogonally. It is intended that two'i'ndependent small motors (approximately one-half horsepower) drive the feed with a positioning accuracy of 0.01 inches, with the system being anti-backlash, reversible and containing limit switches. Automatic tracking is implemented through coupling of the feed positioner motor drives to the drive outputs of the closed-loop servo control unit which uses signal strength input data. Each of the two feeds for dualbeam operation is positioned independently by means of separate positioner/servo control subsystems. Each feed and associated positioner is supported on a rigid shelf attached to the main feed/subreflector support tower, as shown in FIG. 9A.

The geometry of the spherical double reflector configuration lends itself particularly well to location of the electronic equipment units at the rear of the subreflecv tor for the following reasons: this permits a reasonably short run of waveguide from the feed to the electronic equipment input; and this provides negligible r.f. blockage because the electronic equipment is entirely in the shadow of the subreflector.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. An antenna arrangement comprising a spherical main reflector surface, a spherical subreflector surface having a radius of curvature origin coincident with that for the main reflector, and at least one feed predeterminably'arranged relative to said main and subreflector surfaces to illuminate said subreflector surface.

2. The antenna arrangement of claim 1 wherein in the case of a plurality of feeds being provided, each of said feeds is positioned relative to a particular portion of said subreflector surface.

3. The antenna arrangement of claim 1 wherein said spherical subreflector surface is provided by a plurality of spherical subreflectors each concentrically arranged relative to said main reflector surface and positioned a predetermined separation from a spherical center point defined by said spherical main reflector surface, and

wherein a feed is provided for each of said plurality of spherical subrelfectors in effecting multiple-beam operation.

4. The antenna arrangement of claim 1 wherein said main reflector surface constitutes a spherical torus reflector and said subreflector surface constitutes a Cassegrain type spherical torus subreflector.

5. The antenna arrangement of claim 1 wherein in the case of providing a pair of feeds for dual-beam operation, the arrangement of said feeds relative to said main and subreflector surfaces is such as to provide a beam separation range of from approximately 2 to at least 50.

6. The antenna arrangement of claim 5 wherein at least one of said plurality of feeds is adjustably positioned relative to said main and subreflector surfaces along a predetermined circular are for providing the capability of continuous beam scanning in a predetermined plane without requiring movement of either of said main or subreflector surfaces.

7. In a multiple-beam double-reflector antenna arrangement, the combination comprising a sphericalsegment main reflector, at least one sphericalsegment subreflector surface concentrically arranged relative to said main reflector, and'a plurality of feeds predeterminably arranged relative to said main and subreflectors, each feed providing a beam for illuminating the subreflector surface.

8. The antenna arrangement of claim 7 wherein a plurality of spherical-segment subreflectors are provided in one-to-one correspondence with said plurality of feeds.

9. The antenna arrangement of claim 7 wherein said feeds constitute feed horns predeterminably positioned in terms of aperture relative to said main and subreflectors.

10. The antenna arrangement of claim 7 wherein for each feed provided in excess of one, a corresponding angular extension to both the main and subreflector surfaces is provided as defined by the angular separation between said feeds;

11. The antenna arrangement of claim 10 wherein for dual feeds being provided and arranged to provide a 6 beam separation, a corresponding angular addition of 6 to both the main and subreflectors is provided as related to the antenna spherical center point.

12. An energy radiating arrangement comprising a spherical main reflector surface defining a spherical center point 0, at least one spherical subreflector surface positioned a predetermined separation from said center point relative to said main reflector surface and arranged to be concentric therewith, at least one feed optimally positioned relative to said main reflector and subreflector, the entire geometry of the arrangement being concentric with respect to said spherical center point.

13. The antenna arrangement of claim 12 wherein said spherical center point 0 and said spherical main reflector define a main reflector radius R constituting the principal axis of the antenna arrangement, and wherein with a single spherical subreflector and feed being provided, said subreflector and feed are positioned to be centered on said main axis respectively at radii R and R as defined by 0 R R and R R R in which the location R of said feed is optimized in terms of antenna operating efficiency in dependence on the R,:R radius relationship of said main and subreflectors.

14. The arrangement of claim 12 wherein said spherical center point 0 and said spherical main reflector define a radius R constituting the principal axis of the antenna arrangement, with said at least one spherical subreflector being centered thereon at a predetermined radius R R relative to said center point 0 and wherein in the case of the antenna arrangement being provided with a plurality of feeds, said feeds are symmetrically arranged about said axis at a common radius R R R relative to said center point 0, in which R is selected to optimally position said feeds in terms of antenna phase error efficiency relative to the radius relationship R zR of said main and subreflectors.

15. The arrangement of claim 12 wherein said spherical center point 0 and said spherical main reflector define a radius R constituting the main axis of the antenna arrangement, and wherein in the case of the antenna arrangement being provdied with a plurality of spherical subreflecting surfaces and a corresponding plurality of feeds, said subreflectors and associated feeds are arranged about said main axis respectively at radii R and R as defined by 0 R R and R R R in which R is selected to optimally position said feeds in terms of antenna phase error efficiency relative to the radius relationship R zR of said main and subreflectors.

16. In an antenna system, the combination of a spherical main reflector surface which defines a spherical center point and a radius R and a spherical subreflector surface, the radius of curvature origin thereof being coincident with that of the main reflector and which is predeterminably positioned at a radius R R relative to said center point, and further comprising at least one feed positioned, in terms of antenna efficiency, to be optimally arranged for illuminating said subreflector at a predetermined separation from the vertex of said subreflector as a function of the radii R and R 17. A double-reflector antenna arrangement comprising a fixed spherical main reflector of predetermined physical dimensions composed of a multiplicity of reflecting panels having substantially identical spherical reflecting surface curvature arranged in mosaic form to effect a substantially continuous spherical main reflecting surface, at least one spherical subreflector mounted to be concentrically arranged relative to said main reflector at a predetermined separation therefrom, and a separate feed for each subreflector predeterminably mounted relative thereto to form a feed/- subreflector assembly arrangement requiring only feed positioning to implement beam steering and tracking; said feed/subreflector assembly constitutes a Cassegrain type feed/spherical subreflector assembly, and wherein the main and subreflectors are concentric spherical surfaces relative to a common spherical center so as to provide beam pointing as a function of the position of the feed on a spherical radius relative to the main spherical surface; and the total surface area required for said spherical main reflector for multiple beam operation is determined as a function of antenna aperture and maximum beam separation only and independent of the number of beams.

18. A method of optimizing a spherical/spherical double-reflector radiating arrangement, comprising the steps of determining, in relation to a spherical main reflector, the position of a concentrically arranged spherical subreflector in terms of the spherical center point defined by the main reflector and placing the subreflector thereat, wherein the upper limit of the position of the spherical subreflector relative to the spherical main reflector center point is determined by the maximum allowable blocking loss which in turn is further defined by the subreflector-to-main reflector diameter ratio, and wherein the'lower limit is determined by the requirement that blocking due to the feed of the antenna not exceed that of the subreflector; and determining the feed location relative to the vertex of the spherical subreflector for a given F/D ratio so as to provide substantially zero path-length error proximate the aperture edge of the spherical main reflector, wherein the optimum feed to subreflector separation is primarily a function of the position of the subreflector relative to the main reflector spherical center point, with second order dependence on the F/D ratio. 

1. An antenna arrangement comprising a spherical main reflector surface, a spherical subreflector surface having a radius of curvature origin coincident with that for the main reflector, and at least one feed predeterminably arranged relative to said main and subreflector surfaces to illuminate said subreflector surface.
 2. The antenna arrangement of claim 1 wherein in the case of a plurality of feeds being provided, each of said feeds is positioned relative to a particular portion of said subreflector surface.
 3. The antenna arrangement of claim 1 wherein said spherical subreflector surface is provided by a plurality of spherical subreflectors each concentrically arranged relative to said main reflector surface and positioned a predetermined separation from a spherical center point defined by said spherical main reflector surface, and wherein a feed is provided for each of said plurality of spherical subrelfectors in effecting multiple-beam operation.
 4. The antenna arrangement of claim 1 wherein said main reflector surface constitutes a spherical torus reflector and said subreflector surface constitutes a Cassegrain type spherical torus subreflector.
 5. The antenna arrangement of claim 1 wherein in the case of providing a pair of feeds for dual-beam operation, the arrangement of said feeds relative to said main and subreflector surfaces is such as to provide a beam separation range of from approximately 2* to at least 50*.
 6. The antenna arrangement of claim 5 wherein at least one of said plurality of feeds is adjustably positioned relative to said main and subreflector surfaces along a predetermined circular arc for providing the capability of continuous beam scanning in a predetermined plane without requiring movement of either of said main or subreflector surfaces.
 7. In a multiple-beam double-reflector antenna arrangement, the combination comprising a spherical-segment main reflector, at least one sphericalsegment subreflector surface concentrically arranged relative to said main reflector, and a plurality of feeds predeterminably arranged relative to said main and subreflectors, each feed providing a beam for illuminating the subreflector surface.
 8. The antenna arrangement of claim 7 wherein a plurality of spherical-segment subreflectors are provided in one-to-one correspondence with said plurality of feeds.
 9. The antenna arrangement of claim 7 wherein said feeds constitute feed horns predeterminably positioned in terms of aperture relative to said main and subreflectors.
 10. The antenna arrangement of claim 7 wherein for each feed provided in excess of one, a corresponding angular extension to both the main and subreflector surfaces is provided as defined by the angular separation between said feeds.
 11. The antenna arrangement of claim 10 wherein for dual feeds being provided and arranged to provide a theta * beam separation, a corresponding angular addition of theta * to both the main and subreflectors is provided as related to the antenna spherical center point.
 12. An energy radiating arrangement comprising a spherical main reflector surface defining a spherical center point 0, at least one spherical subreflector surface positioned a predetermined separation from said center point relative to said main reflector surface and arranged to be concentric therewith, at least one feed optimally positioned relative to said main reflector and subreflector, the entire geometry of the arrangement being concentric with respect to said spherical center point.
 13. The antenna arrangement of claim 12 wherein said spherical center point 0 and said spherical main reflector define a main reflector radius R1 constituting the principal axis of the antenna arrangement, and wherein with a single spherical subreflector and feed being provided, said subreflector and feed are positioned to be centered on said main axis respectively at radii R2 and R3 as defined by 0<R2<R1 and R2<R3<R1, in which the location R3 of said feed is optimized in terms of antenna operating efficiency in dependence on the R1:R2 radius relationship of said main and subreflectors.
 14. The arrangement of claim 12 wherein said spherical center point 0 and said spherical main reflector define a radius R1 constituting the principal axis of the antenna arrangement, with said at least one spherical subreflector being centered thereon at a predetermined radius R2<R1 relative to said center point 0 and wherein in the case of the antenna arrangement being provided with a plurality of feeds, said feeds are symmetrically arranged about said axis at a common radius R2<R3<R1 relative to said center point 0, in which R3 is selected to optimally position said feeds in terms of antenna phase error efficiency relative to the radius relationship R1:R2 of said main and subreflectors.
 15. The arrangement of claim 12 wherein said spherical center point 0 and said spherical main reflector define a radius R1 constituting the main axis of the antenna arrangement, and wherein in the case of the antenna arrangement being provdied with a plurality of spherical subreflecting surfaces and a corresponding plurality of feeds, said subreflectors and associated feeds are arranged about said main axis respectively at radii R2 and R3 as defined by 0<R2<R1 and R2 <R3<R1, in which R3 is selected to optimally position said feeds in terms of antenna phase error efficiency relative to the radius relationship R1:R2 of said main and subreflectors.
 16. In an antenna system, the combination of a spherical maIn reflector surface which defines a spherical center point and a radius R1, and a spherical subreflector surface, the radius of curvature origin thereof being coincident with that of the main reflector and which is predeterminably positioned at a radius R2<R1 relative to said center point, and further comprising at least one feed positioned, in terms of antenna efficiency, to be optimally arranged for illuminating said subreflector at a predetermined separation from the vertex of said subreflector as a function of the radii R1 and R2.
 17. A double-reflector antenna arrangement comprising a fixed spherical main reflector of predetermined physical dimensions composed of a multiplicity of reflecting panels having substantially identical spherical reflecting surface curvature arranged in mosaic form to effect a substantially continuous spherical main reflecting surface, at least one spherical subreflector mounted to be concentrically arranged relative to said main reflector at a predetermined separation therefrom, and a separate feed for each subreflector predeterminably mounted relative thereto to form a feed/subreflector assembly arrangement requiring only feed positioning to implement beam steering and tracking; said feed/subreflector assembly constitutes a Cassegrain type feed/spherical subreflector assembly, and wherein the main and subreflectors are concentric spherical surfaces relative to a common spherical center so as to provide beam pointing as a function of the position of the feed on a spherical radius relative to the main spherical surface; and the total surface area required for said spherical main reflector for multiple beam operation is determined as a function of antenna aperture and maximum beam separation only and independent of the number of beams.
 18. A method of optimizing a spherical/spherical double-reflector radiating arrangement, comprising the steps of determining, in relation to a spherical main reflector, the position of a concentrically arranged spherical subreflector in terms of the spherical center point defined by the main reflector and placing the subreflector thereat, wherein the upper limit of the position of the spherical subreflector relative to the spherical main reflector center point is determined by the maximum allowable blocking loss which in turn is further defined by the subreflector-to-main reflector diameter ratio, and wherein the lower limit is determined by the requirement that blocking due to the feed of the antenna not exceed that of the subreflector; and determining the feed location relative to the vertex of the spherical subreflector for a given F/D ratio so as to provide substantially zero path-length error proximate the aperture edge of the spherical main reflector, wherein the optimum feed to subreflector separation is primarily a function of the position of the subreflector relative to the main reflector spherical center point, with second order dependence on the F/D ratio. 